1. FIELD OF THE INVENTION
The field of the invention is control of the flow of fluidized solids.
2. DESCRIPTION OF RELATED ART
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425C.-600C., usually 460C.-560C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500C.-900C., usually 600C.-750C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan et al, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not. Although the overall size and height of most of the hardware associated with the FCC unit has decreased in size, the use of all riser reactors has resulted in catalyst and cracked product being discharged from the riser reactor at a fairly high elevation. This elevation makes it easy for a designer to transport spent catalyst from the riser outlet, to a catalyst stripper at a lower elevation, to a regenerator at a still lower elevation. The great "head" afforded by modern designs does increase the head or pressure generated by dense bed fluidized catalyst streams. This usually does not cause too much of a problem when flow control means, such as slide valves, are used to manipulate and control catalyst flows, but can lead to severe problems when an internal valve, such as a ceramic plug valve, is used.
Internal plug valves are used, and indeed essential, for use in modern, compact FCC designs such as the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above. Such a design is compact, efficient, and has a very small "footprint". Because of the compact nature of the design, and the use of a catalyst stripped which is contiguous with and supported by the catalyst regenerator, it is necessary to use an internal means to control spent catalyst flow from the catalyst stripper to the catalyst regenerator, such as a plug valve. Plug valves work, but they are expensive, and subject to a number of problems, as will be evident from the following review of the problems associated with plug valves which was abstracted from U.S. Pat. No. 4,827,967 May 9, 1989 Junier, which is incorporated herein by reference.
Flow control of catalyst from the standpipe into the dense phase of the regenerator from the stripper, and from the regenerator into the riser reactor, is obtained by the use of plug valves engageable with the lower ends of the transfer lines and having elongated valve stems extending through the vessel wall controlled in their longitudinal movement by external mechanical or manual operating means. These plug valves are used in oil refineries in controlling the flow of catalyst into a reaction chamber which is subject to temperature extremes, for example, in the range of 1500.degree. F., as well as in other industrial applications wherein the valves are subject to oppositely directed displacements due to thermal expansion and spring forces.
Plug valves (such as Kellogg Orthoflow Valve, U.S. Pat. No. 2,850,364) are used to control the flow of catalyst to introduce a lift medium such as oil feed stock or lift air into a riser line. One problem occurring with the hollow tube plug valve providing a lift medium through the center hollow section is that the lift medium pressure at the inlet of the valve cannot be maintained at a high enough level to overcome the bottom regenerator pressure. If the regenerator pressure is greater than the lift air pressure, catalyst from the regenerator can block the valve's guide liners and cause the valve to stick. Another problem with prior art valves occurs when the pressure of the lift medium is greater than the regenerator pressure, permitting the lift medium to go between the valve's guide liners causing the valve to stick.
There has been a long-felt need to overcome the problems associated with the prior art plug valves. The present invention addresses and satisfies this long-felt need by eliminating the plug valve, and replacing it with a non-mechanical catalyst flow control means.
Although the most severe problems with plug valves are encountered with those operating inside bubbling dense bed regenerators, there are other places where plug valves are used in FCC units that are also troublesome, such as to control the flow of regenerated catalyst to the riser reactor.
Use of slide valves is common in FCC units when the catalyst stream to be controlled is flowing through a pipe. Slide valves have different problems than plug valves, but are not trouble free. A severe problem is erosion of the slide. Some FCC units end their refinery runs with slide valves in the closed position, which operate as if they were wide open.
The present invention is directed to a novel and efficient way of overcoming the deficiencies of existing technology for controlling the flow of catalyst from a catalyst stripper down through a standpipe to a catalyst regenerator directly underneath the stripper.
The present invention provides a way to control catalyst flow, without the complications and problems that are inherent in using an internal plug valve, namely sealing the plug valve despite the problems of: differentials in regenerator pressure and lift medium pressure; the unwanted sticking of the valves; and the need for excessive amounts of a purge medium.
The present invention provides a better way of controlling catalyst flows around an FCC regenerator, and provides ways of dealing with some of the special problems presented by typical FCC catalyst, namely the unusual settling and flow characteristics of FCC catalyst which make some of the conventional ways of controlling the flow of fluidized solids unsuitable for use in an FCC unit.